1. cache util

利用閉包，每個呼叫cached函式的方法都會生成一個cache變數，在cache變數中做快取

/**
 * Create a cached version of a pure function.
 */
export function cached<F: Function> (fn: F): F {
  const cache = Object.create(null)
  return (function cachedFn (str: string) {
    const hit = cache[str]
    return hit || (cache[str] = fn(str))
  }: any)
}

/**
 * Capitalize a string.
 */
 

export const capitalize = cached((str: string): string => {
  return str.charAt(0).toUpperCase() + str.slice(1)
})

2. reactive property

同樣是閉包，setter更新val的值，getter中通過val取到新的值

/**
 * Define a reactive property on an Object.
 */
export function defineReactive (
  obj: Object,
  key: string,
  val: any
 
,
  customSetter?: ?Function,
  shallow?: boolean
) {
  const dep = new Dep()

  const property = Object.getOwnPropertyDescriptor(obj, key)
  if (property && property.configurable === false) {
    return
  }

  // cater for pre-defined getter/setters
  const getter = property && property.get
 

  const setter = property && property.set

  let childOb = !shallow && observe(val)
  Object.defineProperty(obj, key, {
    enumerable: true,
    configurable: true,
    get: function reactiveGetter () {
      const value = getter ? getter.call(obj) : val
      if (Dep.target) {
        dep.depend()
        if (childOb) {
          childOb.dep.depend()
          if (Array.isArray(value)) {
            dependArray(value)
          }
        }
      }
      return value
    },
    set: function reactiveSetter (newVal) {
      const value = getter ? getter.call(obj) : val
      /* eslint-disable no-self-compare */
      if (newVal === value || (newVal !== newVal && value !== value)) {
        return
      }
      /* eslint-enable no-self-compare */
      if (process.env.NODE_ENV !== 'production' && customSetter) {
        customSetter()
      }
      if (setter) {
        setter.call(obj, newVal)
      } else {
        val = newVal
      }
      childOb = !shallow && observe(newVal)
      dep.notify()
    }
  })
}

3. 取值的一種做法

if (isDef(data) && isDef(i = data.hook) && isDef(i = i.prepatch)) {
    // i = data.hook.prepatch
    i(oldVnode, vnode)
}